As a switching power supply circuit capable of a high speed action, a proposal has been made for the one configured to adjust an output voltage by turning on or off a switching element with the use of the output of a flip-flop circuit, as shown in FIG. 7. As shown in this drawing, this switching power supply circuit is equipped with a comparator 1 which compares a feedback voltage FB based on an output voltage VOUT (i.e., a voltage obtained by dividing the output voltage VOUT by a resistance ratio between the resistances of feedback resistors RFB1 and RFB2) with a reference voltage Vref as the output voltage of a reference power supply V0; a flip-flop circuit 2 which is set by the output of the comparator 1; and an ON-time generation circuit 3 which resets the flip-flop circuit 2 at a time when a predetermined time elapses after an output signal from the flip-flop circuit 2 has fallen. The output signal of the flip-flop circuit 2 is supplied to a switching element (in the present example, a P-channel MOSFET) SW via a buffer circuit 4, whereby the switching element SW is turned on or off. In this manner, the output voltage VOUT is obtained which is a predetermined direct current voltage smoothed by a capacitor CL via a coil L connected between a drain of the switching element SW and a free-wheeling or flywheel diode D.
In the above-mentioned switching power supply circuit, when a pulse signal supplied from an output terminal Q_B of the flip-flop circuit 2 to the switching element SW via the buffer circuit 4 is at an L-level, the switching element SW enters the ON-state. Thus, a coil current ILx flowing through the coil L gradually increases.
When, in this state, an ON-time defined by the ON-time generation circuit 3 has passed, the flip-flop circuit 2 is reset by an ON-time signal outputted from the ON-time generation circuit 3. As a result, the pulse signal supplied from the output terminal Q_B to the switching element SW via the buffer circuit 4 reaches an H-level, and the switching element SW enters the OFF-state. Consequently, the coil current ILx gradually decreases while being bypassed or recirculating via the diode D.
As the coil current ILx decreases, the output voltage VOUT becomes less than the reference voltage Vref. At this time, the flip-flop circuit 2 is set, with the result that the switching element SW enters the ON-state. Consequently, the coil current ILx gradually increases again. Simultaneously, the management or control of the ON-time by the ON-time generation circuit 3 is started. Afterwards, the same actions are repeated.
In FIG. 7, VIN denotes an input voltage, and CFB denotes a speed-up capacitor. A switching power supply circuit similar to that in FIG. 7, is disclosed in Japanese Patent Document JPA-2006-141191.
In the switching power supply circuit according to the conventional technology, as described above, the feedback voltage FB is compared with the reference voltage Vref by the comparator 1, and the flip-flop circuit 2 is set by the output of this comparator 1. By this measure, the on-off control of the switching element SW is exercised. If the ripple component of the output voltage VOUT is small, therefore, switching control in the flip-flop circuit 2 becomes unstable. That is, if the ripple component of the output voltage VOUT is small, the difference of the feedback voltage FB from the reference voltage Vref does not reach a sufficient level. As a result, the timing of setting of the flip-flop circuit 2 deviates from the position on the time base where it should be located. In synchronism with this deviating timing, the following timings also deviate: the timings of the rise and fall of the pulse signal supplied from the output terminal Q_B to the switching element SW via the buffer circuit 4, and the timing of the rise of the ON-time signal fed out via the ON-time generation circuit 3. Consequently, the output voltage VOUT becomes so unstable as to involve undulations or the like.
Such phenomena are more manifest in a case where a capacitor with low ESR (equivalent series resistance) is used as the capacitor CL, or when the switching frequency is increased.
The present invention has been accomplished in the light of the above-mentioned earlier technology. It is an object of the present invention to provide a switching power supply circuit which can be allowed to act stably even when a capacitor with low ESR is used or when the switching frequency becomes high.